1. Field of the Invention
The present invention relates to a method for manufacturing a polishing pad utilized to polish a wafer, a polishing pad, and a method for polishing a wafer.
2. Description of the Related Art
With advancement of high integration of a semiconductor device, a very high level has been demanded with respect to flatness of a semiconductor wafer as a material of the semiconductor device.
When polishing a wafer, e.g., such as a semiconductor wafer, a technique of attaching a polishing pad to an upper side of a turn table and the wafer is slidably contacted to this polishing pad with a polishing slurry interposed therebetween is used.
The polishing pad is roughly classified into a nonwoven type in which a nonwoven fabric is impregnated with polyurethane and a polyurethane type in which polyurethane is foamed. The polyurethane type generally has higher hardness than the nonwoven type. Further, the hardness of the polyurethane type polishing pad is adjusted by changing a composition of polyurethane as a material.
Higher hardness of the polishing pad is desirable to obtain a wafer with high flatness. However, the polishing pad with high hardness has a higher probability of occurrence of a scratch on a wafer. Therefore, appropriately high hardness of the polishing pad must be selected.
On the other hand, a urethane cake as a raw material of a polyurethane type polishing pad has a fluctuation in a density in the cake, and this becomes a local density fluctuation of the polishing pad. The density fluctuation affects flatness of a polished wafer as unevenness of hardness.
When using the polyurethane type polishing pad, a wafer can be flattened and polished with a relatively high accuracy, but this accuracy is still insufficient in terms of a very high flatness level that has been recently demanded, and a yield ratio is also poor.
Although the polishing pad is fixed to a turn table and then a surface treatment (e.g., brushing, seasoning, or dressing) is effected to solve such a problem, flatness of a wafer cannot be sufficiently stably obtained. Further, there is also a problem of loss of a time required to perform such a surface treatment, loss of productivity due to this time loss, or a reduction in a lifetime of the polishing pad.
Furthermore, although a measure for improving the polishing pad itself has been proposed to obtain flatness of a wafer (Japanese Patent Application Laid-open No. 267978-1999 and others), flatness of a wafer cannot be sufficiently stably obtained.